Consider these two headlines, which announce the same study results:
New drug cuts cancer risk by 50%
Drug results in 2% drop in cancer risk
In this hypothetical study described by the Annie Appleseed project, 100 women take a new drug for cancer and 100 women take a placebo. After five years, two women in the first group develop cancer and four in the placebo group develop cancer. Which headline is correct?
They both are, but each uses a different statistic to describe the same study results. Headline one describes the relative risk of developing cancer. Headline two describes the absolute risk.
Relative Risk
Relative risk is the measure of risk of a certain event happening in one group (women taking a drug) compared to the risk of the same event happening in another group (women taking a placebo). Researchers often use relative risk when describing cohort studies (studies that follow a group over time) and clinical trials.
A relative risk of one means there is no difference between the two groups. A relative risk of more than one means (in our example) that women who do not take the new drug are at increased risk of developing cancer. Relative risk, however, is not an increase in risk and it does not tell you anything about your actual risk. Absolute risk helps you do this.
Absolute Risk
Absolute risk is a measure of the risk of a certain event happening, such as the number of people who may develop (or die from) a certain type of cancer within a certain time period. It does not compare risks; it just states the probability that something will happen and does not take into consideration your other risk factors, such as family history. Your absolute risk of developing cancer increases with age.
The frequently cited statistic that one in eight women will be diagnosed with breast cancer represents a woman's lifetime risk of developing breast cancer, assuming she lives 80 or 90 years. However, a woman's actual absolute risk of developing breast cancer depends on her age. For example, only one out of every 233 women, or .43%, between the ages 30 and 39 are at risk for developing breast cancer, while one out of 27 (3.7%) of women in their 60s are at risk.
Sources:
National Cancer Institute. "Absolute Risk." Web.
http://www.cancer.gov/dictionary?CdrID=666196
National Cancer Institute. "Relative Risk." Web.
http://www.cancer.gov/dictionary?CdrID=618613
Woloshin, Steven, Schwartz, Lisa M. and Welch, H. Gilbert. "Risk Charts: Putting Cancer in Context." Journal of the National Cancer Institute 94 (11) (2002): 799-804. Web.
http://jnci.oxfordjournals.org/content/94/11/799.full
BreastCancer.org. "Risk of Developing Breast Cancer. Web. 19 October 2011.
http://www.breastcancer.org/symptoms/understand_bc/risk/understanding.jsp
Ryan, Nancy. "Relative Risk Vs. Absolute Risk." Annie Appleseed Project. Web.
http://annieappleseedproject.org/reriveabri.html
George Mason University. "What is the difference between absolute and relative risk?" Stats.org. Web. http://www.stats.org/in_depth/faq/absolute_v_relative.htm
